1. Field of the Invention
The present invention relates to a method of designing a printed circuit board which can suppress emitted noise that may be generated thereby, and a printed circuit board which suppresses the generation of such emitted noise.
2. Related Background Art
When power-supply patterns and ground potential (to be referred to as ground hereinafter) patterns are to be formed on a printed circuit board, a multilayer printed circuit board is often used. In general, in the multilayer printed circuit board, power-supply and ground patterns are formed as inner layers, while electronic parts and the like are mounted as surface layers to form signal patterns.
Power-supply and ground patterns are generally formed in empty spaces on a double- or single-sided printed circuit board in which no signal patterns are formed.
When patterns are to be wired across other patterns, through holes are formed in the respective patterns, and jumper lines are inserted into the through holes and soldered thereto, thereby electrically connecting the respective patterns.
Power-supply and ground patterns to be formed on a conventional printed circuit board are designed without any special consideration as to the relationship between the frequency of the signal as an emitted noise source and the length of each pattern.
Since the power-supply and ground patterns vary in length and area, the inductances of the patterns vary. In some case, therefore, the inductance of a given portion is considerably larger than that of another portion. When a current flows in this large-inductance portion, the potential varies. As a result, high-level emitted noise may be generated.
In addition, when signals are exchanged between active elements such as an IC and an oscillator mounted on a printed circuit board, and a current flows in a pattern connecting these elements, a magnetic field is generated around the pattern. If the pattern in which the current is flowing has inductance, a potential difference is produced between the pattern and another pattern to generate an electric field. The generated magnetic and electric fields are spread/radiated outword, and the emitted noise caused by these magnetic and electric fields adversely affects other signal patterns, resulting in phenomena such as reflection, crosstalk, and delays.
According to conventional printed circuit boards, in order to prevent adverse effects of such emitted noise, resistors are inserted in patterns, and inductors and capacitors are inserted in patterns to cut high-frequency components.
This method of preventing noise by attaching the above parts afterward will prolong the development period and cause an increase in cost because of changes in design.
Under the circumstances, a method of effectively mounting capacitors for noise prevention by making improvements in the layout of patterns formed on a printed circuit board is disclosed in, e.g., Japanese Patent Publication No. 1-47032.
FIG. 17 shows the layout of patterns formed on a conventional printed circuit board. FIG. 18 is an enlarged view showing the main part of the pattern layout in FIG. 17.
As shown in FIG. 17, a plurality of parallel ground lines 121 are formed on the lower surface of the printed circuit board, and a plurality of power-supply lines 122 are formed parallel to the ground lines 121. A plurality of parallel ground lines 131 are formed on the upper surface of the printed circuit board to extend in a direction perpendicular to the ground lines 121 and the power-supply lines 122 formed on the lower surface of the printed circuit board. In addition, a plurality of parallel power-supply lines 132 are formed parallel to the ground lines 131.
This printed circuit board has a plurality of through holes 150 to allow ICs to be mounted on the board.
Referring to FIG. 18, conduction through holes 134 are formed in the intersections of the ground lines 121 on the lower surface and the ground lines 131 on the upper surface. The ground lines 121 on the lower surface are connected to the ground lines 131 on the upper surface through these conduction through holes 134.
Similarly, conduction through holes 136 are formed in the intersections of the power-supply lines 122 on the lower surface and the power-supply lines 132 on the upper surface. The power-supply lines 122 on the lower surface are connected to the power-supply lines 132 on the upper surface through these conduction through holes 136.
Capacitors (not shown) are inserted in the power-supply and ground lines at predetermined positions. These lines are connected to each other in an AC manner through the capacitors, thereby preventing high-frequency noise.
When, for example, a 16-pin DIP (Dual In-line Package) type IC is to be mounted on such a printed circuit board, the power-supply pin (16th pin) of the IC is inserted in a power-supply through hole 110 in FIG. 18, and the ground pin (8th pin) of the IC is inserted in a ground through hole 111 in FIG. 18, thereby supplying power to the IC.
When, however, the performance of a conventional printed circuit board like the one described above was tested, the emitted noise was not reduced in spite of the use of noise prevention capacitors.
This is because, a noise prevention capacitor is not inserted between the power-supply pin and ground pin of the same IC but is inserted between the power-supply pin and ground pin of different ICs.
That is, in the conventional printed circuit board, power-supply and ground lines are formed at optimal intervals for connection with the power-supply pin and ground pin of an IC (in a DIP type general logic IC, these pins are located on two ends in the longitudinal direction), and capacitors are arranged.
In recent years, however, the operating frequencies of ICs is becoming higher. For this reason, if power-supply and ground lines are arranged at intervals near the length of an IC in the longitudinal direction, high-level electromagnetic waves are radiated from the signal lines or the power-supply lines. The high-level electromagnetic waves may adversely affect the self-circuit or other circuits to make them malfunction.